1. Field of the Invention
The present invention pertains to a condensation control system for heated glass and heated insulating glass units, and more particularly, to a sheet of low emissivity glass with a resistive coating which is connected to a power source. An optical moisture sensor is positioned between the two sheets of glass to detect moisture on the outer surface of the glass caused by condensation. When used in an insulating glass unit for commercial freezer and refrigeration doors, the sensor controls the selective heating of the glass to prevent condensation from forming on the doors.
2. Summary of Related Art
Insulating glass units used in glass doors for commercial freezers and refrigerators are double-paned or triple-paned construction. The insulating glass units generally include a conductive coating on one of the glass surfaces for electrically heating the glass. Heating the glass keeps the doors free of frost and condensation so that customers can see the products in the freezer or refrigerator. The clear glass doors improve sales and keep the frost and condensation from damaging the goods for sale and the cooling equipment.
Because the surface temperature of a glass door is reduced below ambient temperature by the refrigerated interior, moisture tends to condense out on the surface of the glass when the temperature of the glass drops below the dewpoint of the air in the store. The object of heating the glass is to maintain the temperature of the glass above the dew point temperature of the warmer ambient air. By heating the glass above the dew point, the undesirable condensation and frost are prevented from forming on the glass of the door.
In a door consisting of a two-paned insulating glass unit, an unexposed surface of one or both of the sheets of glass is coated with a conductive material. The conductive coating is connected to an alternating current power supply by two bus bars or other electrical connectors mounted on opposite edges of the glass. As is current passes through the coating, the surface of the glass is heated to provide a condensation-free surface.
The coating in an insulating glass unit door is normally applied to the unexposed surface of the frontmost glass sheet. The frontmost sheet of glass, even though it is exposed to the ambient air, can be kept free from frost and condensation. In a humid environment when the door is opened, the innermost sheet of glass is also exposed to the ambient air and condensation may form on the exposed surface of the innermost door. Consequently, the unexposed surface of the innermost sheet of glass may also be coated to heat the innermost sheet of glass.
The current may be transferred through the coating on the glass on a continuous basis. In order to minimize the increase in cost caused by the heat from migrating to the freezer or refrigerator cabinet, the doors are generally constructed of triple pane units for freezers and double pane units for refrigeration coolers. The units are typically operated at a low heat dissipation.
Control of the power dissipated by the freezer or refrigeration door is an important concern. If the power is too low, condensation and frost will form on the glass. If the power dissipation is too high, additional costs will be incurred. The additional energy required to heat the door is a nominal cost, but the operating costs on the cooling system to maintain the freezer or refrigerator at the desired temperature can be significant. In general, the goal is to keep the units free from frost and condensation with a low power dissipation density.
Heated glass may also be used in other applications to prevent condensation, such as vending machines, bathroom mirrors, or skylights. Such units could also include a control system for selectively transmitting current across the coated surface of the glass when condensation is detected.
Sheets of glass suitable for heated applications are provided with a transparent, conductive coating on one surface. Typical transparent conductive coatings include tin oxide, indium oxide, and zinc oxide. The coating on the sheet of glass has a resistance, which is typically measured in "ohms per square," which is the resistance of a square piece of glass.
Sheet resistance in ohms per square is a well known term in the art and is used in accordance with such meaning. For a square piece of coated glass having a known sheet resistance, the resistance between opposing sides of the square piece of coated glass remains constant for any size of square. The resistance can be measured by using a 4 point probe ohmmeter or other similar measuring device.
The coated glass used in the applications noted above is often rectangular in shape. The resistance between opposing sides of the rectangular piece of coated glass varies depending on the dimensions of the glass. However, once the sheet resistance in ohms per square of a specific type of coated sheet of glass is known, the resistance between opposite sides of any rectangular piece of glass can be calculated based on the actual dimensions of the rectangular sheet of glass per the following equation: EQU R.sub.G =(d/w)R.sub.S
where R.sub.G is the resistance of the rectangular piece of coated glass as measured between the opposing sides on which the bus bars are mounted, d is the distance between the two sides with bus bars, w is the length of the two sides on which the bus bars are mounted, and R.sub.S is the surface resistance in ohms per square of a square piece of the coated glass. The ratio of d/w is often referred to as the aspect ratio.
Assuming that the coating is applied in a uniform thickness, the resistance will be uniform across the coated glass. The resistance of the coated glass can also be changed by varying the thickness of the coating applied on the glass. For coated glass connected directly to a power supply, the power dissipation may be controlled by varying the resistance of the coated glass.
A common size for a freezer door is 6 feet by 2 feet. For such a freezer door with a coating having a resistance of 100 ohms per square, the resistance of the freezer door would be 300 ohms measured between the 2-foot sides and 33.33 ohms measured between the 6-foot sides.
When current is continuously applied to the coating on the glass freezer door, the preferred power dissipation density for a freezer door in a humid environment typically ranges from 4-10 watts per square foot. The power dissipation density is reduced for less humid applications, such that the preferred range, in general, is from 1 to 10 watts per square foot. Power dissipation densities above 10 watts per square foot will not generally place undue thermal stress on the coated glass, but will result in inefficient operation of the overall cooling system. For a 2.times.6 freezer door with a desired power dissipation of 6 watts per square foot to heat the door, the total power dissipation for the door is 72 watts. The power dissipated by the door can be controlled by setting the voltage, current, and/or resistance in the system used to heat the door (power=VI=V.sup.2 /R=I.sup.2 R.sub.G).
For a 2.times.6 door with bus bars directly connected to 115 volt power with dissipation density of 6 watts per square foot and power dissipation of 72 watts, the resistance of the coating on the glass door needs to be 183.7 ohms. The coating in ohms per square to achieve the desired resistance depends on which side the bus bars are positioned. If the bus bars are positioned along the short sides, the required ohms per square should be 61.2. If the bus bars are positioned on the long sides of the door, the ohms per square of the coating should be 551. The required coating varies depending on the size of the door and the positioning of the bus bars.
In the production of freezer doors and refrigerator doors for direct connection to a power supply, it has not generally been possible to specify a single coating for the glass produced for the doors. Differences in glass door size, power dissipation requirements, line voltages, and mounting configurations necessitates a number of different coatings with different ohms per square resistances. Because doors required varying sheet resistances, the majority of the glass for the doors are coated in an off-line customized production process in order to provide the resistance matching requirement.
In an off-line production operation, conductive coatings of tin oxide have traditionally been applied to glass using a pyrolytic spray batch process in a re-heat furnace. The sheet resistance is selected to provide the proper power dissipation for the door size and line voltage. The pyrolytic process is well suited to provide the relatively high sheet resistance required for direct connection to a power line. However, such a process has a number of problems. The coating of glass with tin oxide in an off-line process results in high costs, poor uniformity, interference colors which degrade the appearance of the coated glass, and overspray to the opposite surface.
On the other hand, glass coated with tin oxide in a high volume, on-line production operation provides a lower cost and readily available product that has improved clarity, uniformity, and heat transfer properties. Glass producers with high volume production lines for low emissivity glass often use a coating process consisting of atmospheric chemical vapor deposition (ACVD) to produce architectural window glass. Such glass has low hemispheric emissions which improves the insulating properties of the glass. A low emissivity glass (also called low E glass) can also be manufactured by off-line batch spray and off-line vacuum coating. Pyrolytic low emissivity glass produced in an on-line process often includes one or two color suppression layers to suppress the unwanted color of sprayed tin oxide. In an on-line pyrolytic production process, the coating is applied while the glass is being manufactured. The coating equipment is located in the tin bath in the float glass process where the glass is formed such that the residual heat of the glass is used to facilitate the chemical reaction for the coating process.
In multi-paned insulating glass units, such as freezer doors, the glass of the insulating glass unit must be heated to eliminate condensation, but yet have good insulating properties to minimize heat transfer to the freezer cabinet. The goal is to provide a coating on the glass with a low hemispheric emissivity and with a high insulating value (R value). Uncoated glass has a hemispherical emissivity of 0.84, and freezer doors must typically be triple pane units in order to minimize heat transfer into the freezer cabinet. Depending on the thickness, glass coated in an off-line process will typically have a hemispheric emissivity of between 0.4 and 0.8 while a low emissivity coated glass can achieve an improved emissivity in the range of 0.05 to 0.45.
Emissivity is a measure of both absorption and reflectance of light at given wavelengths. It is usually represented by the formula: Emissivity=1-reflectance of the coating. The term emissivity is used to refer to emissivity values measured in the infrared range by ASTM standards. Emissivity is measured using radiometric measurements and is reported as hemispherical emissivity and normal emissivity.
A triple paned insulating glass door constructed with uncoated glass will have an insulating R value of 2.94. A triple paned door with coated glass having an hemispheric emissivity of approximately 0.45 will have an improved R value of 3.70. Using a low emissivity glass of 0.15 emissivity would improve thermal performance such that a lower cost double paned unit could be provided for freezer doors. Such a double paned unit (0.15 emissivity, 0.5 inch air space) will have an R value of 3.33. Adding argon gas between the panes increases the R value to 4.0.
The use of a single low emissivity glass produced on a high volume production line could provide freezer and refrigerator door manufacturers with significant benefits. The cost of the coated glass for use in the heated doors would be reduced significantly and the thermal performance of the glass would be improved. The use of a low emissivity glass with a standard coating for high volume production is the key to obtaining the cost savings.
However, there is a significant problem with the use of low emissivity glass for heated glass applications. The low emissivity glass has low resistance such that the continuous direct connection of the glass to a power supply will produce too great a power density. In addition, the resistance matching requirements have hampered such an application.
The control systems which rely on temperature and/or humidity sensing have not provided acceptable results. Such sensors do not directly detect condensation on the surface of the sheet of glass, and provide only an approximation of when condensation will form. Such systems do not have the sensitivity or accuracy needed for controlling the power to a coated sheet of glass in insulating glass applications. A low cost control system with acceptable performance capabilities is needed to permit the use of the low emissivity glass in insulating glass door units.
Applicants have developed an insulating glass unit with a capacitively coupled heating system for continuous operation. A capacitor is coupled between the power supply and the coating of the glass to provide the desired current reduction and power dissipation for continuous operation. A single type of low emissivity glass can be used for a variety of door sizes and power supplies by changing the capacitance in the control circuit. The details of the coated glass and the control system are disclosed in the co-pending U.S. application Ser. No. 08/779,470, which disclosure is incorporated herein by such reference.
The present invention involves a control system which utilizes an optical sensor for the direct detection of condensation instead of the indirect temperature and relative humidity method. The optical sensor provides improved detection of condensation of the surface of the glass to facilitate the intermittent application of power to the coated glass.
A variety of control systems have been developed in the prior art for heated glass applications and insulating glass units. Transformers have been used to reduce the line voltage to heated glass, as shown in U.S. Pat. No. 4,248,015 to Stromquist et al. Transformers are an unacceptable solution because they are bulky and expensive. External ballast resistors (Also shown in '015) have been used, but these are large and generate unwanted heat.
Transformers have also been used to overcome a problem which frequently occurs when using a coated glass with a fixed resistance directly connected to a power source. If the humidity at an installation might be higher than was expected when the system was designed, possibly due to seasonal variation, the power density of the doors might be insufficient to keep condensation from occurring. Because power density was set by a fixed sheet resistance of the glass, expensive boost transformers have been installed to increase the voltage in order to correct condensation problems.
Control systems have been developed using triac circuits to vary the voltage applied to a heated sheet of glass, an example of which is shown in U.S. Pat. No. 4,260,876 to Hochheiser. Hochheiser senses the difference in the surface temperature of the glass and the dew point temperature of the ambient air and uses a complex solid state switch to control the current. Complex triac phase control circuits, however, may cause loads to the power line that have high peak currents and high harmonic content. Additionally, triac circuits cause large amounts of electromagnetic interference (EMI). Triac circuits which reduce harmonic distortion and EMI have been taught, for example, by Callahan et al. in U.S. Pat. No. 5,319,301. Such triac circuits, however, are complex, expensive, and of only limited effectiveness in reducing peak currents.
Reiser et al. (U.S. Pat. No. 5,347,106) discloses a control system for heating a mirror to prevent condensation formation. The coating is split into separate conductive elements with one or more scribe lines in order to control the length of the conductive path. Heaney, in U.S. Pat. No. 4,127,765, teaches that several doors may be wired in series.
Heaney also discloses the use of sensors to detect the ambient temperature, the dew point, and the relative humidity for controlling power to the coated glass. In an earlier patent (U.S. Pat. No. 3,859,502), Heaney disclosed a relative humidity sensor and a controller for controlling power to the glass based on the level of relative humidity. Humidity is sensed based on variable impedance of resistive component or electrode. Sensors for detecting relative humidity and/or temperature are disclosed in U.S. Pat. Nos. 4,277,672; 4,350,978; and 4,827,729. Temperature sensors alone, where the glass is maintained at a specified temperature, do not provide accurate control since the dew point is dependent upon both temperature and humidity. The systems with both temperature and humidity sensors have not provided the accuracy or the response time needed to efficiently operate the insulating glass units with heated glass.
Technology has been developed in the automotive industry for sensing moisture on a windshield to control the automatic operation of the windshield wipers. Wiper control systems have employed a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive (detecting variable impedance), capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam being diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing (i.e. disturbances in an optical path) is directly related to the phenomena observed (i.e., disturbances in the optical path that effects the vision, which in this case is condensation observed by the person at the door of the freezer). Thus, optical systems generally have an advantage over other sensor technologies in that they are closely related to the problem corrected by the wipers in a windshield or by the heating of the glass in an insulating glass door unit.
McCumber et al. (U.S. Pat. No. 4,620,141) disclose an automatic control circuit for triggering a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield. The rain sensor devices for controlling the windshield wipers of a vehicle as disclosed by McCumber et al. and Teder (U.S. Pat. Nos. 5,059,877 and 5,239,244) include a box-like housing mounted upon the interior surface of the windshield. The presence of moisture on the surface of the windshield changes the reflection of light at the air-glass interface, and this change in reflected light is electronically processed and utilized as the signal for activating the windshield wipers.
In the present invention, optical sensors can provide improved detection and control for eliminating condensation and facilitating the use of the low emissivity glass. The sensor housing in an optical moisture sensor should securely engage the glass and be optically coupled to the glass so as to effectively eliminate the interface between the light emitters-detectors and glass surface from an optical standpoint. In optical moisture sensors, light from an emitter is directed by a guide means into the glass at an angle of approximately forty-five degrees with respect to the glass. The light is then reflected by the outer surface of the glass at approximately a forty-five degree angle and is directed by a guide means into a detector. Water or other condensation on the surface of the glass effects the overall transmittance of the optical path between emitter and detector.
When the angle of entry of the light beam into the glass is greater than fifty degrees, a loss of signal frequently occurs. When the angle of entry is less than forty degrees, a loss of sensitivity occurs and the sensor is not able to properly detect moisture on the glass. Consequently, it is essential that the angle of entry of the light beam from the emitter enter the glass at approximately forty-five degrees.
Examples of optical sensor mounting configurations to achieve the forty-five degree angle between the optical axis of the emitter and the glass windshield are disclosed in Noack (U.S. Pat. No. 4,355,271), Bendicks (U.S. Pat. No. 5,323,637), Larson (U.S. Pat. No. 4,859,867), and Stanton (U.S. Pat. No. 5,414,257). Teder, one of the present applicants, has a co-pending application (U.S. Ser. No. 08/653,546, incorporated herein by reference) which discusses the configuration of an optical sensor in further detail.
In addition to the mounting of the optical moisture sensors on the glass, various control circuits have been developed in the windshield wiper application for processing signals from an optical moisture sensor and generating a control signal. Teder (U.S. Pat. No. 5,059,877) involves a control circuit for a windshield wiper system which is designed to drive the wiper blades at a rate dependent on the level of precipitation encountered but which also addresses the problem of noise associated with shifts in ambient light level.
Several automotive glass applications use less expensive conductive sensors instead of optical sensors. The conductive sensors are formed on the sheet of glass to measure the moisture on the surface of the glass and are based on the principle that moisture located on the surface between two electrodes will vary the impedance between the two electrodes. U.S. Pat. Nos. 3,902,040; 4,032,745; and 4,127,763 describe electrical systems used in the automotive glass industry for heaters. U.S. Pat. No. 3,968,3342 describes a coated automotive glass with a pair of electrodes attached to the surface of the glass sheet. Current through the coated glass is turned on and off in response to the variations in electrode resistance caused by condensation. The electrodes are mounted on the outer surface of the sheet of glass such that the electrodes frequently corrode or deteriorate in use, which adversely effects the accuracy of the control system.
In summary, the cost, complexity, and other problems associated with sensors and power conversion circuits have prevented manufacturers from achieving the cost benefits of using a standard low emissivity glass in heated insulating glass for freezer doors and other applications. Optical sensors provide an accurate system for moisture detection, but such optical sensors have not been used in conjunction with low emissivity coating of glass in an insulating glass unit.